6.6. Indexed mode

The Indexed command converts your image to
indexed mode. See
indexed colors in the
Glossary for more information about Indexed Color Mode.

6.6.1. Activating the Command

You can access this command from the image menubar through
Image → Mode → Indexed.

6.6.2. The “Convert Image to Indexed Colors” dialog

The Indexed command opens the
Convert Image to Indexed Colors dialog.

Figure 16.63. The “Convert Image to Indexed Colors” dialog

Colormap Options

Generate optimum palette:
This option generates the best possible palette with a
default maximum number of 256 colors (classic GIF format).
You can reduce this
Maximum Number of Colors, although
this may create unwanted effects (color banding) on smooth
transitions. You may be able to lessen the unwanted effects
by using dithering, however.

Use web-optimized palette: use a
palette that is optimized for the web.

Use black and white (1-bit) palette:
This option generates an image which uses only two colors,
black and white.

Use custom palette:
This button lets you select a custom palette from a list.
The number of colors is indicated for each palette. The
“Web” palette, with 216 colors, is the
“web-safe” palette. It was originally created
by Netscape to
provide colors that would look the same on both Macs and
PCs, and Internet Explorer 3 could manage it. Since
version 4, MSIE handles a 212 color palette. The problem
of color similarity between all platforms has not been
solved yet and it probably never will be. When designing a
web page, you should keep two principles in mind: use light
text on a dark background or dark text on a light
background, and never rely on color to convey information.

Some colors in the palette may not be used if your image
does not have many colors. They will be removed from the
palette if the Remove unused colors from final
palette option is checked.

Dithering Options

Since an indexed image contains 256 colors or less, some colors
in the original image may not be available in the palette. This
may result in some blotchy or solid patches in areas which
should have subtle color changes. The dithering options let you
correct the unwanted effects created by the Palette Options.

A dithering filter tries to approximate a color which is
missing from the palette by instead using clusters of pixels
of similar colors which are in the palette. When seen from a
distance, these pixels give the impression of a new color. See
the Glossary for more information on
dithering.

Three filters (plus “None”) are available. It is
not possible to predict what the result of a particular filter
will be on your image, so you will have to try all of them and
see which works best. The
“Positioned Color Dithering” filter is well
adapted to animations.

Figure 16.64. Example: full color, with no dithering

This is an example image with a smooth transition in
RGB Mode.

Figure 16.65. Example: four colors, with no dithering

The same image, after being transformed to four indexed
colors, without dithering.

Figure 16.66. Example: Floyd-Steinberg (normal)

The same image, with four indexed colors and
“Floyd-Steinberg (normal)” dithering.

Figure 16.67. Example: Floyd-Steinberg (reduced color bleeding)

The same image, with four indexed colors and
“Floyd-Steinberg (reduced color bleeding)”
dithering.

In a GIF image, transparency is encoded in 1 bit: transparent
or not transparent. To give the illusion of partial
transparency, you can use the Enable dithering of
transparency option. However, the
Semi-flatten
plug-in may give you better results.

Note

You can edit the color palette of an indexed image by using the
Colormap Dialog.